The method known by the name “Smart Cut®” is used to detach a thin film and to transfer it onto a support, sometimes called a stiffener, by performing the following steps:
1. bombardment of one face of an initial substrate with gaseous species or ions (H or rare gases), in order to implant those ions (or atoms) in a concentration sufficient to create a layer of microcavities,
2. bringing of this face of the substrate into intimate contact (typically by molecular bonding) with a second substrate called the support or stiffener,
3. fracturing of the layer of microcavities by the application of a heat treatment and/or a detachment stress (for example, the insertion of a blade between the two substrates and/or application of traction and/or bending and/or shear forces and/or application of ultrasound or of microwaves of judiciously chosen power and frequency), and
4. recycling of the substrate.
In the case of the production of heterostructures, for example in the case of the transfer of a thin film of a material A onto a substrate of material B, if the step 2 of bringing the two substrates into intimate contact is followed by a heat treatment (consolidation of the intimate contact (bonding) or thermal fracture), unbonding or rupture of the two bonded substrates can be observed. Because of their intrinsic properties, the different materials A and B generally have different coefficients of thermal expansion (CTE). The more different the CTE, the less readily can the integrity of the bonded structure be maintained at high temperatures. Accordingly, in the case of a self-supporting GaN substrate of approximately 325 microns thickness (denoted ˜325 μm) bonded to a sapphire substrate of thickness ˜330 μm, the two bonded substrates must not in practice be heated to a temperature beyond approximately 230° C. (i.e. not beyond ˜230° C. using the above notation): beyond 230° C., unbonding of the two substrates is observed, i.e. their detachment from each other at the bonding interface. This low temperature resistance is particularly problematic for the fracture step 3, since this step generally consists in whole or in part of a heat treatment, and heat treatments are in practice conducted at much higher temperatures. The bonding temperature resistance therefore limits the fracture heat treatment.
It is known from U.S. Pat. No. 5,877,070 (primarily concerning silicon, silicon carbide, germanium or diamond) that a sensitization step by heat treatment (at high temperature) of the implanted plate before the bonding step reduces the subsequent fracture heat treatment. The problem is that this sensitization step is of limited effect: it must not induce deformation of the surface in the form of blisters or even exfoliated areas. This in practice implies that the sensitization cannot represent more than about 10% of the fracture thermal budget; as a result of this the fracture treatment proper, after bonding to the stiffener, must therefore rely on approximately 90% of the fracture thermal budget. This limited sensitization therefore does not enable a significant reduction in the fracture treatment as such, with the result that the limitation imposed by the bonding temperature resistance remains even after such sensitization treatment. It is appropriate to mention here that the fracture thermal budget corresponds to the annealing time necessary to produce the fracture for a given annealing temperature (it is clear that the fracture time depends on the annealing temperature); the fracture thermal budget depends on the implantation conditions, notably on the nature of the ions (or atoms) implanted, their dose, their energy, the substrate implanted, etc.
The step 2 of bringing the implanted substrate into intimate contact with the stiffener must in principle be effected with plane and perfectly clean surfaces. The problem is that, when it takes place, this bringing into intimate contact cannot be produced effectively over all of the surface of the substrates:
firstly, the edges of the plates forming the substrates are generally chamfered and therefore cannot be brought into contact; this problem of non-bonding at the edge of plates is encountered for all materials (Si, Ge, GaAs, GaN, sapphire, SiGe, LiTaO3, LiNbO3, SiC, InP, etc.) and for all plate diameters between 5 cm and 30 cm (in practice between 2 inches and 12 inches);
in the case of substrates or layers structured intentionally (for example by patterns produced photolithographically) or unintentionally (for example by growth defects in the case of epitaxial layers or by defects linked to the deposition of a layer on the initial substrate), patterns or defects that are recessed in the surface give rise to non-bonded areas (NBA);
finally, in the case of insufficiently effective cleaning, the presence of particles (“dust”) at the bonding interface also gives rise to NBA.
In the fracture step, if the dimension of the NBA is large relative to the thickness of the film to be transferred (for example with a ratio (NBA lateral dimension)/(film thickness) of approximately 10), the thin film remains locally fastened to the initially implanted substrate. These areas are called non-transferred areas (NTA).
These bonding defects can even lead to highly localized (over dimensions of a few square microns) lifting or even detachment of the thin film, in the form of blisters or exfoliated areas, which has to be avoided completely.
By way of example, FIG. 1 represents a substrate 1, here of sapphire, onto which a layer 2, here of GaN, has been deposited and has then undergone implantation that has resulted in the formation of an implanted area 2A. On this layer 2 is represented an optional bonding layer 3. This layer 2 is in intimate contact at an interface 9 with another substrate 4, here also of sapphire, and also provided with an optional bonding layer 5, for example similar to the layer 3.
It is seen that, the substrates being chamfered, the peripheral areas P are not bonded. Moreover, because of a defect that has occurred when depositing the GaN layer, there is a recess in the bonding layer 3 and therefore a non-bonded area C. Finally, the reference I represents dust trapped between the bonding surfaces (remaining after ineffective cleaning) and locally reducing or even eliminating the mechanical strength of the bonding interface 9.
It is seen in FIG. 2 that, at the time of fracture in the implanted area, there remain non-transferred areas in vertical alignment with the peripheral areas, the growth defect and the dust.
According to a notable advantage of the “Smart Cut®” technology, the initially implanted substrate can, after peeling a thin film during the fracture step, be recycled for other, analogous transfer cycles. However, it has just been seen that the thin film remains locally attached to the substrate in the non-transferred areas, therefore forming steps with a thickness typically between 10 and 1000 nm (corresponding to the thickness of the thin film). Moreover, implantation followed by fracture generally cause roughness to appear at the bared surface of the substrate. It follows from this that recycling the substrate from which a thin film has just been detached generally necessitates particular leveling steps, notably by mechanical polishing and/or chemical attack (this is explained in particular in the documents EP-A-1 427 002 and EP-A-1 427 001).